Recently, with the development of products such as a semiconductor laser, a Charge Coupled Device (CCD) and a Liquid Crystal Display (LCD), research into a technical field using volume holographic digital data storage technology has been actively carried out. While a fingerprint recognition system for storing and reproducing fingerprints has been put to practical use as a result of this research, it trends toward extension into a variety of fields that can utilize the advantages of large-capacity storage capability and an ultra-high data transfer rate.
A holographic digital storage and reproduction system is an apparatus that records an interference pattern, which is generated when a signal light from an object interferes with a reference light, in a storage medium, such as a photorefractive crystal, that sensitively reacts with the amplitude of the interference pattern. As the storage and reproduction system records the amplitude and phase of a signal light with changing the angle of the reference light or the like, it can store several hundred to thousand holograms in the same location, wherein each of the holograms is a page composed of binary data.
Meanwhile, in a recording mode in which hologram data is recorded in a storage medium, a typical holographic digital storage and reproduction system divides laser light, which is generated by a light source, into reference light and object light; modulates the object light into pages of binary data in which pixels constitute light and darkness in accordance with external input data (i.e., input data to be stored); and records an interference pattern in the storage medium as hologram data corresponding to the input data, wherein the interference pattern is obtained by causing modulated object light (i.e., a signal light) and reference light, which is divided and reflected at a predetermined deflection angle, to interfere with each other.
Meanwhile, hologram data is read from a storage medium in a reproduction mode. And, the read hologram data, i.e., a reproduced data image, may have the problems of variation in magnification, such as the enlargement or reduction of a reproduced data image, and pixel misalignment, which are due to an error in an optical system, the surface status of the storage medium, or a focusing error of an objective lens that focuses the reproduced data image. A typical and conventional method for solving the above-described problems is an oversampling technique.
FIG. 9 is a block diagram of a holographic reproduction system that employs the conventional oversampling technique. The holographic reproduction system includes a spindle motor 902, a storage medium 904, a reading light path 906, a reproduced light path 908, an image detection block 910, a border detection block 912 and an oversampling block 914.
With reference to FIG. 9, the holographic reproduction system is provided with the storage medium 904 rotated by the spindle motor 902. And, the storage medium 904 is provided with the reading light path 906, along which reading light necessary to reproduce recorded hologram data is irradiated onto the storage medium 904; and the reproduced light path 908, along which data image light (i.e., a checker-shaped pattern of binary data) reproduced by the irradiation of the reading light is output, wherein the reproduced light path 908 includes an objective lens for focusing the data image light.
Furthermore, the image detection block 910, such as a CCD camera, is provided on a terminal of the reproduced light path 908. The CCD camera photoelectrically converts the reproduced data image light in the manner of oversampling each of the pixels thereof into n×n pixels (e.g., 3×3 pixels), and provides the converted result to the border detection block 912. In this case, the data image light read from the storage medium 904, i.e., a reproduced image frame, includes a data image region and a border region. For example, if it is assumed that the reproduced image frame has a reproduced data image with the resolution size of 240×240 and has 3-pixel-sized upper, lower, right and left borders, the image detection block 910 generates a photoelectrically converted reproduced image frame which includes borders and has a resolution size of 1024×1024, and the photoelectrically converted reproduced image frame is provided to the border detection block 912.
Thereafter, the border detection block 912 detects the border region with a method of, e.g., referring to the total brightness of pixels with respect to each line of the converted reproduced image frame; extracts an oversampled reproduced data image from the reproduced image frame, based on information about the detected borders; and transfers the extracted oversampled reproduced data image to the oversampling block 914. For example, if a reproduced image frame with a size of, e.g., 1024×1024 is obtained from a reproduced data image having a resolution size of 240×240 and 3-pixel-sized upper, lower, right and left borders, with converting a pixel into 3×3 pixel, the border detection block 912 extracts an oversampled reproduced data image having a resolution size of 720×720 and transfers it to the oversampling block 914.
And, the oversampling block 914 extracts an original reproduced data image from the oversampled reproduced data image through sampling using a 3×3 mask. That is to say, for example, the oversampling block 914 extracts an original data image (i.e., an encoded data image) having a resolution size of 240×240 in such a way that one pixel is extracted and then two pixels are skipped from an oversampled data image having a resolution size of 720×720 (i.e., a method in which a center pixel is extracted from each of 3×3 masks), as shown in FIG. 10. The original data image extracted as described above is provided to a decoder (not shown) for decoding. In FIG. 10, n1 to n4 indicate the 3×3 mask sections along the horizontal direction.
In this case, when the size of the oversampled data image is, e.g., increased due to distortion attributable to various external factors, for example, when a data image that should have had a resolution size of 720×720 has a resolution size of 722×722, the oversampling block 914 divides an entire data section (i.e., a data line) into three equal parts along each of horizontal and vertical directions, and performs sampling in such a way that three, instead of two, pixels are skipped at the start positions of the second and third equal parts (i.e., equal division compensation method).
However, the conventional method using the oversampling technique so as to prevent the deformation (distortion) of a reproduced data image, which occurs due to an error in an optical system, the surface state of a storage medium, and a focusing error of an objective lens for focusing a reproduced data image has a problem in that a CCD must be designed to be larger than necessary. This problem acts as a factor to impede the light weight, compact size and low price of the holographic reproduction system.
Furthermore, the conventional method using the oversampling technique must process a reproduced data image of an unnecessary large size, thereby causes a problem in data reproduction rate.
Moreover, in the convention method, when the resolution size of a reproduced data image becomes greater than that of an actual data, the distortion of pixels are compensated for in such a way that a data line is divided into equal parts, the number of which is identical to (the number of increased pixels+1), and one pixel is additionally discarded at each of the start points of the equal parts. In this case, considering that the probability that the coordinates of the border of the data image falls exactly on an integer value is very low, a problem arises in that distortion cannot be correctly compensated for, so that the problem makes it difficult to attain a high quality reproduced image.
Meanwhile, a hologram data, i.e., the reproduced data image reproduced from the storage medium, may exhibit a phenomenon in which the entire image is linearly/nonlinearly distorted due to a lens error of an optical system (i.e., error caused by a difference in the refractive index in the horizontal and vertical directions of the lens), an error in the grating of a hologram data, an error attributable to the horizontal failure of a storage medium disk and the like. However, the conventional method using the oversampling technique does not consider the linear/nonlinear distortion of the entire image caused by the above-described various factors. In the conventional method, oversampling is performed while the top left, that is, the first pixel of first line of the data image is regarded as a reference position, and the first pixels of every other lines are assumed to be aligned with the reference position. Accordingly, when the linear/nonlinear distortion of the image is serious, a problem arises in that the position of a target pixel that is extracted through oversampling can be misaligned, and the problem acts as a factor to degrade the quality of the reproduced image.